The invention relates generally to systems for finding lost objects. In particular, the invention relates to paired devices, particular those employing acoustic search signals, for finding the lost one of the pair.
It is common in personal and business life to lose small items in a relatively small area. A common example is a set of personal keys including keys for a personal automobile and probably for home and business. Automobile manufactures invariably supply duplicate sets of keys. The car owner usually carries one set in a pocket or purse and leaves the other set in a known location such as a key rack or storage container. If the primary set is misplaced, typically in process of changing clothes or emptying pockets or purses, the other, secondary set is usually readily available. However, the key owner usually wants to find the primary set, both because of the other keys attached to the key ring but also to assure that the secondary set is not subsequently also lost. Typically, the lost set of keys is known to be in a relatively restricted area, for example, at home in one of two or three rooms, since the lost keys were most probably used to drive home and open the door. Thereafter, their whereabouts in the home may be uncertain. As a result, the search for the missing keys needs to cover only a limited area, but the owner is usually in a rush to leave and wants to find them immediately. Similar limited-area searching is often required for eyeglass cases, television remotes, security badges, and the like.
A lost item finding system may comprise two or more identical or nearly identical locators. Transmission of a search signal from one of the locators causes one or more of the other locators to emit a beacon signal, such as an audible signal and a flashing light, enabling the user to locate the lost locator and attached items. The locators may substantively differ only in a programmed identification code used for either transmitting and/or receiving the search signal.
A preferred embodiment uses an acoustic transducer, for example, a piezoelectric transducer, to receive an acoustic search signal, to transmit a corresponding acoustic search signal, and to transmit an audible beacon signal. In transmission mode, the piezoelectric transducer may be subjected to bipolar pulsing across its two inputs. In reception mode, one input is left floating and connected to amplifying, pulse shaping, and counting circuitry while the other input is held at a fixed potential.
The acoustic transducer may be coupled to a Helmholtz resonant acoustic cavity tuned to the resonant frequency of the piezoelectric element, preferably having an annular output port coupled to a cylindrical cavity. A cap may both define the end of the cavity and be spaced from the case enclosing the cavity to form the annular output port.
Preferably, each locator can be selectively disabled, allowing two locators to perform the responding and beaconing without interference from additional locators.
The acoustic search signaling transmission and detection may be arranged to minimize the effects of ambient noise to allow operation in particularly noisy environments. The search signal may consist of a sequence of tones having well defined frequency differences and separated by quiet periods. The detection of the search signal on one hand may require the detection of a quiet period following the tone sequence but on the other hand may respond to multiple transmissions of the tone sequences in a noisy environment.
The baseline frequency, the first tone of the tone sequence, may preferably be any frequency in a broad range, and the detection circuitry nonetheless can detect the tone sequence by determining the frequency differences of the tones relative to the baseline frequency.